Spectrally resolved transmission imaging has been achieved using undetectable infrared light around 1550nm for the first time. A nonlinear interferometer based on spontaneous parametric down-conversion successfully imaged attenuation within a zebrafish larva; previously such detailed biological imaging required expensive and cooled detectors. This silicon-based method attained a resolving power of approximately 1460 at that wavelength, the finest reported to date with undetected photons, over a spectral window spanning 51cm$^{-1}$.

The team pioneered a new imaging technique using undetectable infrared light; key to this advancement is avoiding the need for expensive, cooled detectors traditionally required for such work. The method transfers information from these wavelengths into visible light via a process called nonlinear interferometry and successfully visualised details within a zebrafish larva. Achieving finer detail than previously possible with undetected photons opens avenues for more accessible biological research and broader applications of quantum sensing technologies.

Infrared imaging holds immense potential for distinguishing materials and biological tissues due to how molecules absorb light at specific wavelengths; however, current detection technology remains expensive and cumbersome. Traditional infrared sensors require significant cooling and offer limited pixel counts compared to readily available silicon cameras. To circumvent these limitations, researchers are increasingly turning to nonlinear interferometry, a technique that transfers information from invisible infrared light into visible ranges detectable by standard silicon detectors.

This process can be likened to breaking a dollar bill into smaller bills: lower energy photons representing the original infrared signal are created through a special crystal interaction. Now, spectrally resolved transmission imaging using undetectable infrared light around 1550nm has been achieved with an unprecedented resolving power of approximately 1460, the finest reported for this type of quantum sensing; but how does this complex setup translate into clearer biological images.

Doubled Infrared Imaging Resolution via Silicon Interferometer Calibration

Previously limited to resolving powers around ~730 when imaging with undetected photons, the system achieved a breakthrough resolution of approximately 1460, doubling precision. Careful calibration of their silicon-based nonlinear interferometer enabled this leap; it surpasses existing limits in spectrally resolved transmission imaging without directly detecting infrared wavelengths. By successfully mapping attenuation within a live zebrafish larva, researchers validated its potential for biological applications and opened new avenues for quantum sensing technologies that circumvent costly cryogenic cooling systems.

Achieving spectral resolution across an instantaneous window of roughly 51cm$^{-1}$ further establishes this technique as an important advancement towards practical infrared imaging solutions. A peak baseline visibility of around 74% was attained in near-infrared interference fringes, indicating high contrast within the system. The demonstrated instantaneous spectral window, the range of infrared wavelengths analysed simultaneously, measures approximately 51cm$^{-1}$, allowing broad spectrum analysis at once.

Mapping reductions in light intensity inside a live zebrafish larva proved viability for biological studies; it also confirmed penetration through living tissue while maintaining single-pair photon generation with less than 3.7x 10 to the power of minus six photons per electromagnetic mode. While these results represent strong progress, current spatial resolution remains limited by the one millimetre full width half maximum idler beam waist and improving focus is key to achieving truly detailed images.

Recovering Image Data From Previously Undetectable Infrared Photons

This silicon-based method offers an exciting alternative to expensive cooled detectors for infrared imaging, but registering attenuation alone isn’t enough, a more complex image must be produced. The team acknowledges that a one millimetre spot restricts detailed observation due to limitations in current spatial resolution caused by beam size. Demonstrating spectrally resolved transmission mapping creates contrast from subtle differences in light absorption; it proves information can be gleaned using undetected photons.

Subtle absorption differences now create contrast within samples, validating this breakthrough as infrared imaging with previously considered lost photons. Researchers at Indian Institute of Technology Bombay and Raman Research Institute demonstrated the use of nonlinear interferometry, combining beams to reveal hidden properties about a signal alongside spontaneous parametric down-conversion which converts difficult-to-detect infrared wavelengths into visible light registered by standard cameras. This technique opens doors for affordable thermal sensing, potentially beginning a new era in the field.

This research successfully created spectrally resolved transmission images using undetectable photons around 1550nm. By employing a folded nonlinear interferometer and spontaneous parametric down conversion, researchers were able to map variations in light absorption within a sample, in this case, a zebrafish larva, without directly detecting infrared radiation.

The system achieved spectral resolution of approximately 51 cubic centimetres inverse with resolving power up to 1460, representing the finest reported imaging performance utilizing undetected photons; improving beam focus is noted as an area for future refinement. This method provides an alternative approach to traditional infrared detection that does not rely on expensive cryogenic equipment.

👉 More information
🗞 High Resolution Hyperspectral Quantum Infrared Imaging with Undetected Photons
✍️ Ananya Asthana, Priyanka Kandankel, Sreelaja Nair and Anshuman Kumar (Indian Institute of Technology Bombay); Tanmoy Chakraborty (Affiliation: Raman Research Institute); Aadi Suketu Bharatia and Vidya Prakash Muppalla
🧠 ArXiv: https://arxiv.org/abs/2610.32292

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